The invention relates to the fields of computer-assisted dentistry and orthodontics.
Two-dimensional (2D) and three-dimensional (3D) digital image technology has recently been tapped as a tool to assist in dental and orthodontic treatment. Many treatment providers use some form of digital image technology to study the dentitions of patients. U.S. patent application Ser. No. 09/169,276 describes the use of 2D and 3D image data in forming a digital model of a patient""s dentition, including models of individual dentition components. Such models are useful, among other things, in developing an orthodontic treatment plan for the patient, as well as in creating one or more orthodontic appliances to implement the treatment plan.
The inventors have developed several computer-automated techniques for subdividing, or segmenting, a digital dentition model into models of individual dentition components. These dentition components include, but are not limited to, tooth crowns, tooth roots, and gingival regions. The segmentation techniques include both human-assisted and fully-automated techniques. Some of the human-assisted techniques allow a human user to provide xe2x80x9calgorithmic hintsxe2x80x9d by identifying certain features in the digital dentition model. The identified features then serve as a basis for automated segmentation. Some techniques act on a volumetric 3D image model, or xe2x80x9cvoxel representation,xe2x80x9d of the dentition, and other techniques act on a geometric 3D model, or xe2x80x9cgeometric representation.xe2x80x9d
In one aspect, the invention involves obtaining a three-dimensional (3D) digital model of a patient""s dentition and analyzing the model to determine the orientation of at least one axis of the model automatically. In some implementations, the model""s z-axis is found by creating an Oriented Bounding Box (OBB) around the model and identifying the direction in which the OBB has minimum thickness. The z-axis extends in this direction, from the model""s bottom surface to its top surface. Moreover, in a dentition model having only one mandible, one of the model surfaces is substantially flat and an opposite surface is textured. The direction of the positive z-axis can be identified in this type of model by identifying which of the surfaces is flat or textured. One technique for doing so involves creating one or more planes that are roughly normal to the z-axis and then creating line segments that extend between the planes and the top and bottom surfaces of the dentition model. The surface for which all of the line segments are of one length is identified as being the flat surface, and the surface for which the line segments have varying lengths is identified as being the textured surface.
In other implementations, the x- and y-axes are found by selecting a two-dimensional (2D) plane that contains the axes and an arch-shaped cross section of the dentition model and identifying the orientations of the axes in this plane. In general, the arch-shaped cross section is roughly symmetrical about the y-axis. One technique for identifying the y-axis involves identifying a point at each end of the arch-shaped cross section, creating a line segment that extends between the identified points, and identifying the orientation of the y-axis as being roughly perpendicular to the line segment. The point at each end of the arch can be identified by selecting a point that lies within an area surrounded by the arch-shaped cross section, creating a line segment that extends between the selected point and an edge of the 2D plane, sweeping the line segment in a circular manner around the selected point, and identifying points at the ends of the arch-shaped cross section at which the sweeping line segment begins intersecting the cross section of the dentition model and stops intersecting the cross section of the dentition model. In general, the x-axis is perpendicular to the y-axis.
In another aspect, the invention involves using a programmed computer to create a digital model of an individual component of a patient""s dentition by obtaining a 3D digital model of the patient""s dentition, identifying points in the dentition model that lie on an inter-proximal margin between adjacent teeth in the patient""s dentition, and using the identified points to create a cutting surface for use in separating portions of the dentition model representing the adjacent teeth.
In some implementations, 2D cross sections of the dentition model are displayed to a human operator, and the operator provides input identifying approximate points at which the interproximal margin between the adjacent teeth meets gingival tissue. In some cases, the dentition model includes a 3D volumetric model of the dentition, and the input provided by the operator identifies two voxels in the volumetric model. The computer then defines a neighborhood of voxels around each of the two voxels identified by the human operator, where each neighborhood includes voxels representing the dentition model and voxels representing a background image. The computer selects the pair of voxels, one in each neighborhood, representing the background image that lie closest together.
In some of these implementations, the computer also identifies voxels on another 2D cross section that represent the interproximal margin. One technique for doing so is by defining a neighborhood of voxels around each of the selected voxels, where each neighborhood includes voxels representing the dentition model and voxels representing a background image, projecting the neighborhoods onto the other 2D cross section, and selecting two voxels in the projected neighborhoods that represent the inter-proximal margin.
In another aspect, the invention involves displaying an image of a dentition model, receiving input from a human operator identifying points in the image representing a gingival line at which a tooth in the dentition model meets gingival tissue, and using the identified points to create a cutting surface for use in separating the tooth from the gingival tissue in the dentition model. The cutting surface often extends roughly perpendicular to the dentition""s occlusal plane.
In some implementations, the cutting surface if created by projecting at least a portion of the gingival line onto a plane that is roughly parallel to the occlusal plane and then creating a surface that connects the gingival line to the projection. One way of establishing the plane is by fitting the plane among the points on the gingival line and then shifting the plane away from the tooth in a direction that is roughly normal to the plane. For example, the plane can be shifted along a line segment that includes a point near the center of the tooth and that is roughly perpendicular to the plane. The length of the line segment usually approximates the length of a tooth root.
In other embodiments, the cutting surface extends roughly parallel to the dentition""s occlusal plane in the dentition model. In some of these embodiments, the input received from the human operator identifies points that form two 3D curves representing gingival lines at which teeth in the dentition model meet gum tissue on both the buccal and lingual sides of the dentition model. The cutting surface is created by fitting a surface among the points lying on the two curves. For each tooth, a point lying between the two curves is identified and surface triangles are created between the identified point and points on the two curves. One technique for identifying the point involves averaging, for each tooth, x, y and z coordinate values of the points on portions of the two curves adjacent to the tooth.
Other embodiments involve creating, for each tooth, a surface that represents the tooth""s roots. One technique for doing so involves projecting points onto a plane that is roughly parallel to the occlusal plane and connecting points on the two curves to the projected points. The surface can be used to separate portions of the dentition model representing the tooth roots from portions representing gingival tissue. The model of the tooth roots is then connected to the tooth model.
Other embodiments and advantages are apparent from the detailed description and the claims below.